The present invention relates to a weighing cell for an electronic balance with two sidewalls which are displaced parallel to each other as a function of a weight, in which two metallic plates are disposed, forming a capacitor with capacitances changing as a function of weight.
Such weighing cells are known from DE 29 14 449 C2.
A capacitive transducer is known from DE 29 48 165 A1. It consists of a rigid diaphragm body and a displaceable plate disposed above it. A movable diaphragm part, which is displaced by pressure in a pressure chamber, is provided inside the rigid diaphragm body. It supports the plate by means of spacing beads, so that the flexible part is displaced when the pressure chamber is charged with pressure. Flat areas of a conducting material are placed by vacuum evaporation on the flexible diaphragm part. Flat areas are also placed on the plate by vacuum evaporation. They respectively form an outer and an inner electrode pair, which change their capacitance differently when the diaphragm part is displaced. An output signal is derived from this change when the pressure chamber is charged with pressure. The non-displaceable part of the diaphragm has electrical switching elements on an extension. The complicated construction is disadvantageous in this arrangement, as well as the fact that it is necessary to dispose a flexible diaphragm part, on which it is intended to place different flat areas by means of vacuum evaporation, inside a rigid and non-displaceably disposed diaphragm body. In addition, the transfer of this arrangement to other arrangements is very difficult, because in DE 29 48 165 the pressure chamber must be carefully sealed. The object of the invention, also in respect to this arrangement, is to provide a considerably simplified and, in regard to its volume, more compact unit of weighing cell and evaluation electronics.
It is an object of the present invention to provide a weighing cell of this type which is even more compact and simple, particularly with the view that the entire balance, i.e. inclusive of the associated electronic components, can be constructed smaller and simpler.
This object is attained in accordance with the present invention in that it is provided, in a weighing cell with two sidewalls which are displaced parallel to each other as a function of a weight, in which two metallic tabs are disposed, forming a capacitor with capacitances changing as a function of weight, that one plate is a printed circuit board which is provided with a metallic layer on the side facing the other plate and which furthermore supports the components of the electrical switching device of the balance.
Thus, in accordance with the present invention it is possible to use one plate of the capacitor already as a printed circuit board which contains the components of the evaluation and display circuit. In this way a great amount of space is saved. In contrast to the known device, this plate is not rigidly disposed, but is a part of the movable capacitor arrangement. It is not necessary to provide the plate with a flexible diaphragm part, as in the prior art, instead it is itself rigid, but movable.